1. Field of the Invention
This invention relates to a multi-layered film having a polyolefin layer and at least one polyester layer adjacent to the polyolefin layer. More specifically, it relates to a multi-layered film from which a polyolefin single-layer film or a polyester single-layer film can be easily peeled and which can be cast at a high speed.
2. Description of the Prior Art
To produce a plastic film by a molten film formation technique, a molten polymer is extruded into a film and then cooled over a rotary cooling drum (cooling drum) to cast it into a solid unoriented film. This unoriented film is further drawn monoaxially or biaxially into a monoaxially oriented film or a biaxially oriented film as required. At the time when the polymer is cooled over the cooling drum, such inconvenience as the nonuniformity of the film occurs if the polymer is cooled ununiformly. To prevent this, it is necessary to adhere the molten film closely and uniformly to the cooling drum. Techniques for adhering the molten film closely to the cooling drum include one for casting a molten film under application of electrostatic charge to the molten film (JP-B-37-6142) (The term "JP-A" as used herein means an "unexamined" published Japanese patent application and the term "JP-B" as used herein means an "examined publication for patent"), one for adhering a molten film to the cooling drum with mechanical force such as a roll or belt (JP-B-37-8977), one for adhering with air force such as an air knife (JP-B-37-8978), one for casting under application of electrostatic charge to a film of a copolyester prepared by copolymerizing a component having a high dielectric constant, and the like.
When the above techniques are applied in the production of a multi-layered film having a polyolefin layer and at least one polyester film adjacent to the polyolefin layer, various problems are caused by poor adhesion between the polyolefin layer and the polyester layer, high electric resistance of polyolefin and the like.
That is, when a technique for casting while electrostatic charge is applied to a molten film is used, sufficient charge cannot be given to the molten film because polyolefin has a high electric resistance even in a molten state. As a result, adhesion between the molten film and the cooling drum lowers due to small coulomb force therebetween, whereby nonuniformity in thickness, peeling between layers and displacement between layers occur, resulting in such problems as formation of extremely unstable film. When a technique for casting while electrostatic charge is applied to a polyester prepared by copolymerizing a component having a high dielectric constant is used, similar problems arise.
When a technique for adhering with mechanical force such as a roll or belt, such problems as peeling between layers and displacement between layers occur because a force of peeling of the film from the cooling drum exceeds adhesion between the polyolefin layer and the polyester layer.
When a technique for adhering with air force such as an air knife, the molten film strongly vibrates due to an air flow, resulting in producing remarkable nonuniformity in thickness and making it difficult to obtain a desired multi-layered film.
Meanwhile, JP-A-51-30862 discloses a method for producing sheet-like plastic articles, which comprises the steps of co-extruding at least two different plastics which are not compatible with each other into sheets and separating these sheets to prepare single sheet-like plastic article.
However, this publication fails to disclose co-extrusion of polyester and polyolefin as well as cooling over a cooling drum.
JP-A-52-37982 discloses a method for orienting a thin film, which comprises the steps of orienting a laminated film which consists of a base film and a 2 to 10 .mu.m-thick film laminated on the base film and peeling off the thin film from the base film.
However, this publication also fails to disclose a laminated film comprising a polyester and a polyolefin and cooling of an unoriented laminated film over a cooling drum.
JP-A-63-239810 discloses a method for producing a capacitor element, which comprises the steps of vapor depositing a metal on the surface of a polyphenylene sulfide film of a laminated film which consists of a polyolefin or polyester substrate film and a 0.2 to 1.5 .mu.m-thick polyphenylene sulfide film formed on one or both surfaces of the substrate film, and rolling or laminating the metal-vapor deposited film while the metal-vapor deposited polyphenylene sulfide film was peeled off from the laminated film to form a capacitor element consisting of the polyphenylene sulfide film as a dielectric and the metal vapor deposition layers as electrodes, the thickness of the substrate film being in the range of 5 to 100 times the thickness of the polyphenylene sulfide film.
However, this publication fails to disclose a polyolefin containing a quaternary phosphonium salt of sulfonic acid and cooling of the laminated film over a cooling drum.